1. Field of the Invention
The present invention relates to ceramic nanowires with diameters ranging from several to several tens of nanometers and a process for producing such ceramic nanowires.
2. Prior Art
Silicon carbide fibers undergo only small changes in tensile strength and elastic modulus even if they are heated at 1500° C. They are also much better than carbon fibers in oxidation resistance and in addition they are extremely low in reactivity with metals. Hence, silicon carbide fibers are drawing attention as heat- and corrosion-resistant materials, a typical example of which is a reinforcing material to be used in metal-based composites. Such silicon carbide fibers are synthesized by chemical vapor deposition (CVD) or pyrolysis of silicon-containing polymers (the latter being called the “precursor method”).
The CVD process consists of causing a core wire of carbon or tungsten to react with a feed gas of SiC at high temperature so that the latter is deposited on the wire's surface. Silicon carbide fibers produced by this method have high purity and excel in mechanical characteristics and the like; however, their diameter is very thick, on the order of 100 μm, so difficulty has been involved in working them in such a way that they are suitable for use as a reinforcing material in composites.
Being developed by Yajima et al. in 1975, the precursor method is now an industrial process in which the silicon-containing polymer polycarbosilane (PCS) as the starting material is melt spun, then cured and fired. The intermediate step is important in that the molecules in the melt spun polymer fibers are crosslinked with themselves so that they can be converted to ceramic while retaining the fiber's shape. Two commercialized processes of the precursor method are thermal oxidation in which the silicon-containing polymer is heated in an oxidizing atmosphere so that it is crosslinked via oxygen atoms, and EB irradiation in which the silicon-containing polymer is directly crosslinked by irradiation of electron beams. By means of these techniques, various silicon carbide fibers are already commercially available and they include NICALON® and HI-NICALON®.
The fibers produced by the precursor method are in filamentous form having diameters of 10-15 μm. Active efforts are being made today to develop small shaped ceramics for use in micromachines, as well as nanodevices that take advantage of the heat- and corrosion-resistant properties of SiC. In order that the fibers produced by the precursor method are applied as reinforcing fibers in those small shaped ceramics or nanodevices materials, even finer ceramic fibers are needed. Conventional techniques for synthesizing such ultrafine fibers include a focused ion beam (FIB) method that employs a focused ion beam to perform micro-cutting, and a trimming method that depends on photolithography. However, nanowires, or fibers with diameters on the order of nanometers cannot be formed by those methods. Other synthesis methods known in the art include a process in which fine particles of a silicon-containing polymer are dispersed in a thermally degradable polymer, drawn, and fired into ceramic, and a process in which a carbon nanotube is converted into ceramic. However, it is difficult to control the fiber length and thickness to desired values by those processes.
A study of the present inventors has led to a technique of synthesizing nanowires from silicon-containing polymers by means of ion beams (see Formation of Nanowires along Ion Trajectories in Si Backbone Polymers, S. Seki, K. Maeda, S. Tagawa, H. Kudoh, M. Sugimoto, Y. Morita, and H. Shibata, Adv. Mater. 13(2001), 1663-1665). The principle of the technique is shown in FIG. 1. When a thin film formed of the silicon-containing polymer polysilane is irradiated with an ion beam, sufficient energy is deposited on the silicon-containing polymer (e.g. polysilane) along the trajectories of the individual ions penetrating the thin film, whereupon the molecular chains near the trajectories are first cleaved, then recombined (crosslinked). The points of recombination (crosslinking points) are distributed along the trajectory of the ion beam and their density decreases with the radial distance from the center of trajectory. As shown in FIG. 1, this causes a cylindrical crosslinked portion to form in the thin film of the silicon-containing polymer along the ion beam trajectory. By subsequent washing in a solvent that is capable of dissolving the silicon-containing polymer, all part of the polymer film except the cylindrical crosslinked portion is dissolved away by the action of the solvent, thus producing a cylindrical crosslinked nanowire. Taking advantage of this nature, the present inventors set out to study a new method for producing ceramic nanowires.